Such accumulators, in particular piston accumulators, membrane accumulators or bubble accumulators are used in hydraulic systems, for example of excavators and, amongst other things, serve for the storage of hydraulic energy. In this connection, an accumulator can be configured as a cylindrical tube which is provided with a cover both on the fluid side and also on the gas side. The separation element separates the fluid, in particular a hydraulic liquid, such as e.g. oil, from gas, e.g. nitrogen.
If a fluid is now pressed against the gas in the accumulator, the separation element is displaced, whereby the gas is compressed. The gas therefore takes up energy. The filling of the accumulator typically takes approximately 3 to 5 seconds. At a later point in time the so stored energy can be regained by volume extraction.
In this connection the regained energy depends on the efficiency and on the arising losses. The latter can be attributed to the fact that the change of state does not take place adiabatically over the overall period of time, as it is not possible to configure the walls of the accumulator sufficiently insulated due to the high arising pressures. Moreover, gas losses can arise.
During operation the pressure in the gas space is matched to the pressure in the fluid space, so that the separation element is present at a suitable position within the accumulator. Possible differences between the pressure in the fluid space and the pressure in the gas space are attributed to the friction of the separation element.
In the following any volume which is filled with gas is understood as a gas space. The gas space thus includes both the region of the accumulator which is filled with gas and also one or more gas containers which are connected to the accumulator, as well as possible connection lines filled with gas including gas connections, which connection lines are also referred to as gas lines in the following X. The storage volume can be increased simply and cost-effectively by additionally connected gas containers.
In order to ensure a safe operation it is necessary to control the pressure in the gas space from time to time. Thus, for example, one has to expect certain losses of a gas in dependence on the kind of operation of the accumulator. Moreover, the pressure in the gas space is not allowed to undercut a certain minimum value so that the function of the system can be maintained.
It must also be constantly controlled, whether the accumulator can deliver the desired minimum energy. If a plurality of accumulators are switched in parallel it must be checked that no accumulator undercuts a certain minimum pressure. Moreover, faulty accumulators must be identifiable.
It is known to measure the temperature of the gas for the determination of the pressure in the gas space. A precise measurement of the temperature is, however, difficult in particular due to the inhomogeneous distribution of the gas. A determination of the position of the separation element based thereon with the aid of the gas laws is therefore not possible or flawed with large errors.
It is further known to determine the position of the separation element with the aid of sensors. In this way, for example, cable operated measurement systems are used. However, these are limited with respect to the maximum speed of the separation element and are unsuitable for high changes in load and large numbers of load changes.
Alternatively, the position of the separation element can be measured by means of a rod connected to the separation element which rod is guided out of the accumulator. A guided out rod, however, requires additional seals at the rod. Moreover, the additional demand in space is disadvantageous.
Magnetic apparatuses are also known which are attached at the separation element and which transfer information through the accumulator housing to the outside. Such systems are of complex design and for this reason are frequently prone to trouble.
It is further known to use ultrasonic sensors for the determination of the position of the separation element. However, if the formation of gas bubbles is brought about, for example, due to a fast expansion of the fluid, or an inhomogeneous distribution is brought about, for example due to a strong temperature gradient, the propagation of sound is disturbed by scattering and deflection of the sonic signals, whereby the ultrasonic data is distorted.